In extracorporeal blood processing, blood is taken out of a human or animal subject, processed (e.g. treated) and then reintroduced into the subject by means of an extracorporeal blood flow circuit (“EC blood circuit”) which is part of a blood processing apparatus. Generally, the blood is circulated through the EC blood circuit by a blood pump. In certain types of extracorporeal blood processing, the EC blood circuit includes an access device for blood withdrawal (e.g. a so-called arterial needle) and an access device for blood reintroduction (e.g. a so-called venous needle), which are inserted into a dedicated blood vessel access (e.g. fistula or graft) on the subject. Such extracorporeal blood processing includes hemodialysis, hemodiafiltration, hemofiltration, plasmapheresis, etc.
In extracorporeal blood processing, it is vital to minimize the risk for malfunctions in the EC circuit, since these may lead to a potentially life-threatening condition of the subject. Serious conditions may e.g. arise if the EC blood circuit is disrupted downstream of the blood pump, e.g. by a VND event (VND—Venous Needle Dislodgement), in which the venous needle comes loose from the blood vessel access. Such a disruption may cause the subject to be drained of blood within minutes.
VND may be detected during blood processing based on a pressure signal from a pressure sensor (“venous pressure sensor”) on the downstream side of the blood pump in the EC circuit. Conventionally, VND monitoring is carried out by comparing one or more measured static pressure levels with one or more threshold values. However, it may be difficult to set appropriate threshold values, since the static pressure in the EC blood circuit may vary between treatments, and also during a treatment, e.g. as a result of the subject moving. Further, if the venous needle comes loose and gets stuck in bed sheets or the subject's clothes, the measured static pressure level might not change enough to indicate the potentially dangerous situation.
WO97/10013 proposes alternative techniques for VND monitoring based on the venous pressure signal. In one alternative, VND monitoring is based on detection of heart pulses in the pressure signal. The heart pulses represent pressure pulses produced by a patient's heart and transmitted from the patient's circulatory system to the venous pressure sensor via the blood vessel access and the venous needle. An absence of heart pulses in the pressure signal is taken as an indication of a possible VND event.
US2005/0010118, WO2009/156174 and US2010/0234786 disclose similar or alternative techniques of VND monitoring based on detection of heart pulses in the venous pressure signal.
WO2010/149726 discloses techniques for VND monitoring based on detection of physiological pulses other than heart pulses in the venous pressure signal. Such physiological pulses originate from the human subject, e.g. from reflexes, voluntary muscle contractions, non-voluntary muscle contractions, the breathing system, the autonomous system for blood pressure regulation or the autonomous system for body temperature regulation.
In order to provide a consistent and reliable VND monitoring based on heart pulses or other physiological pulses, it is important to ensure that the pressure signal is substantially free from pulsations that may interfere with the detection of the physiological pulses. For example, it is known that strong repetitive pulsations from the blood pump (“pump pulses”) may be present in the pressure signal at a rate similar to the heart pulsations. In this respect, WO2009/156175 proposes techniques for filtering a pressure signal in the time domain for the purpose of eliminating (or suppressing) the pump pulses while retaining the physiological pulses. These techniques involve estimating the shape of the pump pulses, by obtaining a “predicted signal profile”, at the relevant operating condition of the EC blood circuit and by subtracting the predicted signal profile from the pressure signal. In one implementation, a library of predicted signal profiles is recorded from a pressure sensor in the EC blood circuit in a reference measurement before treatment, e.g. during a priming phase or during a simulated treatment, at a plurality of different operating conditions of the EC blood circuit. In another implementation, the library of predicted signal profiles is generated by simulations using a mathematical model of the EC blood circuit. Based on the current operating condition of the EC blood circuit, a predicted signal profile may be selected from the library and used for eliminating the pump pulses. As an alternative to using pre-recorded or pre-calculated signal profiles, WO2009/156175 proposes recording the predicted signal profile during regular operation of the EC blood circuit, specifically by obtaining a pressure signal from a so-called “system pressure sensor” which is located between the blood pump and the dialyzer in the EC blood circuit. If the blood pump is a peristaltic pump, the system pressure sensor may be substantially isolated from the heart pulses, such that its pressure signal contains pump pulses and no heart pulses, or heart pulses that are significantly suppressed. Thus, in this special situation, the predicted signal profile of the pump pulses may be inferred from the pressure signal of the system pressure sensor and used for filtering the pressure signal generated by the venous pressure sensor.
The present Applicant has realized that the venous pressure sensor may also be responsive to pressure variations with an origin outside of the EC blood circuit, specifically from a supply system for dialysis fluid which is connected in fluid communication with the dialyzer. Such a supply system typically includes one or more valves and one or more fluid pumps that may generate pressure variations in the dialysis fluid, and these pressure variations are propagated via the blood processing unit into the EC blood circuit, where they may be detected by the venous pressure sensor. Depending on supply system, the pressure variations may take the form of a continuous, more or less randomly varying pressure level, or they may be manifested as distinct pulses that are generated at regular intervals or more irregularly, or a combination of both. Experiments indicate that the pressure variations from the supply system may seriously interfere with the detection of physiological pulses in the pressure signal from the venous pressure sensor.
The Applicant has found it difficult to apply the teachings of aforesaid WO2009/156175 to eliminate or suppress the pressure variations that originate from the supply system. For example, it is non-trivial to utilize a library of predicted signal profiles if the supply system is operated independently of the EC blood circuit and information about the operational state of the supply system is unavailable or incomplete. Furthermore, the use of predicted signal profiles is likely to result in insufficient removal of pressure variations that are non-repetitive or random, no matter if the predicted signal profiles are generated by reference measurements before the treatment, by reference measurements using a system pressure sensor in the EC blood circuit during the treatment, or by simulations. Furthermore, there are EC blood circuits that have no system pressure sensor.
Recently, it has also been shown to be possible to monitor and analyze the behavior of physiological pressure generators such as the heart or respiratory system, based on pressure recordings in the EC blood circuit. Various applications are found in WO2010/149726, WO2011/080189, WO2011/080190, WO2011/080191 and WO2011/080194.
Furthermore, WO2011/080188 proposes a technique for identifying and signaling a reverse placement of the devices for blood withdrawal and blood reintroduction in the vascular access by detecting and analyzing physiological pulses in a pressure signal recorded in the EC blood circuit.
All of these monitoring techniques presume that the physiological pulses can be reliably detected in the pressure signal.